Conventionally, a protection element mounted on a secondary battery device or the like is employed as the one having a function of protection from an overvoltage as well as an overcurrent. This protection element is formed so that: a heating element and a soluble conductor made of a low-melting metal are laminated on a board; and the soluble conductor causes a blowout due to an overcurrent, and if an overvoltage occurs as well, power is supplied to the heating element in the protection element, and the soluble conductor causes a blowout due to a heat of the heating element. The blowout of the soluble conductor is due to the goodness of wettability relative to a surface of a connected electrode at the time of blowout of the soluble conductor which is the low-melting metal; the blown low-melting metal is gravitated onto the electrode; and as a result, the soluble conductor is brown out, and a current is shut off.
On the other hand, with downsizing of electronic devices such as portable devices in recent years, there has been a need for downsizing/thinning of the protection element of this type. Further, there has been a demand for the stability and fastness of its operation. As a means thereof, a soluble conductor of a low-melting metal is disposed on an insulation board, and is sealed with an insulation cover member; and the soluble conductor is coated with a flux. This flux is provided so as to prevent oxidization of a surface of the soluble conductor, and at the time of heating of the soluble conductor, to speedily and stably fuse it.
Such protection element has a structure shown in FIG. 28. In this protection element, a soluble conductor 3 made of a low-melting metal is provided between a pair of electrodes 2, which are formed on both ends of a baseboard 1, and an insulation cover member 4 is provided facing the soluble conductor 3 on the baseboard 1. Further, on the baseboard 1 of this protection element, a pair of electrodes 2, although not shown, are further provided at an opposite edge part orthogonal to the pair of electrodes 2, and a heating element 5 made of a resistor therebetween is provided. The heating element 5 is laminated in proximity to the soluble conductor 3 via an insulation layer 6 and a conductor 7. In addition, the cover member 4 which is mounted on the baseboard 1 is covered forming a predetermined space 8 relative to the soluble conductor 3. Further, the soluble conductor 3 is coated with a flux 9, and the flux 9 is accommodated in this space 8.
In addition, a protection element of which a soluble conductor is sealed with an insulation cover member has a structure disclosed in Patent Document 1. In this protection element, since a space in which a fused metal gathers on an electrode at the time of blowout of the soluble conductor is reduced due to thinning thereof, a metal pattern is provided at a site facing each electrode on an interior face of an insulation cover plate, in order to ensure gravitation of the fused metal to each electrode portion.
Further, as disclosed in Patent Document 2, there is proposed the one in which: in order to prevent dispersion of an operating temperature, a piece of soluble alloy is coated with a flux; and a groove or a glass belt, for preventing the spread wetting of the fused alloy, is provided at the periphery of an electrode to which a soluble alloy is connected.